1. FIELD OF THE INVENTION
This invention relates to the treatment of porous materials especially porous materials that initially have had an oxidic surface. More particularly, this invention is directed to increasing the water-repellency of a masonry surface. This invention is particularly directed to increasing the water-repellency of a masonry surface which has been subjected to carbonic acid or the adverse effects of air thereupon.
2. DISCUSSION OF THE PRIOR ART
The treatment of surfaces made from a porous material to improve the surfaces water-repellency is broadly known. For instance, there is described in Ser. No. 190,684 of Oct. 19, 1971 (now U.S. Pat. No. 3,819,400), the disclosure of which is hereby incorporated by reference, that porous surfaces which have available hydroxyl groups can be treated to have their water-repellency improved by initially subjecting the surface to a hydrothermal treatment as by impinging steam thereagainst to provide a relatively clean surface. The surface is thereafter treated with a silane or siloxane which bonds to the surface to markedly increase the water-repellency thereof. In increasing the water-repellency, a complex chemical reaction takes places which involves the large number of hydroxyl groups present on the surface of the porous material. Usually these hydroxyl groups provide anchorage for the formation of X-O-Si bonds, where X is calcium, aluminum, silicon or titanium or the like.
Conditions are quite different, however, when the hydroxyl groups originally present on the surface of the material have already become chemically altered or reacted in some other way. The causes of such chemical alterations of the surface may not only be from normal aging but can also be from environmental conditions. Normal aging effects include especially the phenomenon of carbonation whereby the usually basic surface area of the porous material to be reacted reacts with the carbonic acid in the atmosphere. The effect of this reaction is that in the affected areas the more or less pronounced basicity of the material, e.g., concrete, is reduced or even destroyed. Although this natural reaction progresses slowly, depending upon the concentration of carbon dioxide in the air, it takes place whenever air has access to the material, especially carbonic acid bearing air. The consequences are undesirable alterations of the elasticity moduli of the boundary regions and of the passivation of any kind of steel that is embedded in the porous material, e.g., concrete.
By special environmental conditions are meant all those which involve gaseous, liquid or solid chemicals which act on the material and react with it chemically, thereby causing free hydroxyl groups to be bonded. Depending on the location of the material, this action can take place relatively rapidly such as, for example, when the material is exposed to combustion gases, acid mists, salt atmospheres and the like. In some cases, for example in the formation of chlorides, it is possible that the aforementioned hydromechanical treatment, especially one in which hot water or steam is used, may cleave the compounds that have formed and restore the desired free hydroxyl groups. In many cases, however, when carbonates or sulfates have formed, for example, this preliminary treatment will not permit this restoration of free hydroxyl groups. Thus, it is seen that the very cleaning operations can, in some cases, adversely affect the chemistry of the surface of the porous material to be treated, whereby there is precluded the restoration of free hydroxyl groups which should be available for the subsequent impregnation with silane.
The environmental influences described, however, not only bring it about that the hydroxyl groups needed for the reaction with the organo silicon compounds react with other substances on the surface to be treated, but also that the normally at least weakly basic environment on the surface areas important to the impregnation is neutralized. Since the effectivness of the silanes and siloxanes used to increase the water-repellency can be attributed at least in part to the fact that hydrolysis takes place in an initial reaction which is then followed by the reactions with the substrate, the shift of the pH value of the areas in question from the basic to the neutral results in a greater reduction of the rapidity of this initial part of the reaction. It is obvious that this interferes with the entire surface treatment system and retards the progress of the complex overall reaction. Hence, it has become desirable to improve the water-repellency process applied to surfaces offering an oxidic or hydroxyl containing surface, whereby the normal decrease in the basicity of the environment of the surface is effectively counteracted. Moreover, it has become desirable to improve the overall water-repellency without substantial additional costs.